1. Field of the Invention
This invention relates to a lubricating oil composition to be suitably used for internal combustion engines.
2. Background Art
An internal combustion engine (hereinafter referred to simply as engine) of an automobile comprises an oil pan arranged at a lower part of the cylinder block of the engine main body for feeding lubricating oil (hereinafter referred to engine oil) stored in the oil pan to various parts of the engine and causing it to circulate for lubrication.
The engine oil stored in the oil pan needs to be totally replaced in the course of time because it becomes degraded as it is used for lubrication.
According to owner's manuals provided by automobile manufacturers, the engine oil of an automobile typically needs to be replaced for every 15,000 km of mileage at best if engine oil of the highest grade such as API SH grade is used.
Therefore, the automobile owner has to remind him- or herself not to forget about periodical engine oil change, taking the mileage of the car into consideration.
On the other hand, engine oil for stationary gasoline and diesel engines also requires periodic replacement at the cost of time and labor that can significantly raise the running cost of the engine.
In view of these circumstances, various so-called maintenance-free systems have been proposed to reduce the cost and labor of servicing by extending the service life of engine oil as much as possible.